gimp

You also can take screenshots with gimp (File -> Acquire -> Screenshot ...).

xwd

xwd is part of the xorg-apps package.

Take a screenshot of the root window:

xwd -root -out screenshot.xwd

scrot

Scrot, available in the "extra" repository, provides for taking screenshots from the CLI, and offers features such as a user-definable time delay. Unless instructed otherwise, it saves the file in the directory bash was at when the command was launched.

scrot -t -d 5

saves a dated .PNG file, along with a thumbnail for Web posting. It provides a five second delay before capturing, in this instance.

KDE

If you use KDE, you might want to use ksnapshot, which can also be activated using <Prt Scr>.

GNOME

You can press <Prt Scr> or Apps->Accessories->Take Screenshot.

Note: if <Prt Scr> complains about not finding gnome-screenshot or there is no "Take Screenshot" entry in your menu, you will need to install the gnome-utils package.